Physics

Electrical Machines and Circuits

754 Questions

Electrical machines and circuits involve the principles of alternating and direct current, transformers, diodes, and power consumption. These concepts are essential for engineering and technical exams. Review these practice questions to test your knowledge of circuit analysis.

Transformers and impedanceAlternating and direct currentCircuit components and diodesPower consumption calculations

Electrical Machines and Circuits Questions

Multiple choice physics effects of electric current electric generator ac generator magnetic effects of electric current

In an AC generator, the ends of the coil are connected to ____________.

  1. two slip rings

  2. the split ring commutator

  3. the external circuit directly

  4. None of these
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The slip rings allow the transfer of alternating e.m.f. induced in the rotating coil to the external circuit. Each ring is connected to one end of the coil wire and is electrically connected to the external circuit via the conductive carbon brushes.

So, In an AC generator, the ends of the coil are connected to two slip rings
Therefore, A is correct option

Multiple choice physics magnetism and electromagnetism electric generator ac generator magnetic effects of electric current

A domestic electrical appliance requires alternating current of 15 V. If 220 V of alternating current is supplied to the house, then the device that helps in the functioning of that electrical appliance is

  1. Induction coil

  2. Step-up transformer

  3. AC dynamo

  4. Step-down transformer
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

A step down transformer is one in which the secondary coil has lesser number of turns in secondary coil than in primary coil.  Due to this the ratio of voltages in the coils is

$\dfrac{V _s}{V _p}=\dfrac{N _s}{N _p}$ 
Hence the voltage across the secondary coil in the step down transformer is reduced.
Therefore such a transformer is used to apply a voltage of $15V$ across a device when alternating voltage of $220V$ is applied.

Multiple choice physics effects of electric current electric generator ac generator magnetic effects of electric current

In an a.c. generator the speed at which the coil rotates is doubled. How would this affect the frequency of output voltage ?

  1. Frequency of maximum output voltage is halved.

  2. Frequency of maximum output voltage is doubled.

  3. Frequency of maximum output voltage remains same.

  4. Insufficient data.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
In an a.c. generator, emf produced is,

    $e=NAB(2\pi v)sin(2\pi v)t$

          where $N=$ no. of turns

                      $A=$ cross- sectional area

                      $B=$ magnetic field.

A frequency of ac, $v=\dfrac{w}{2\pi}$

                    where $w=$ is the speed of rotation

From the above, Frequency of maximum output voltage is directly proportional to speed of rotation of coil.

B. Frequency of maximum output voltage is double.
Multiple choice physics magnetism and electromagnetism electric generator ac generator magnetic effects of electric current

A dynamo is used to supply electric current to a small electric bulb, If the number of turns in the coil of this dynamo is doubled, then the voltage in the bulb.

  1. Will decrease

  2. Remains unchanged

  3. Will be doubled

  4. Will increase by a factor between 1 and 2
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

When the number of turns in the coil increases, the strength of the voltage induced in the coil will also increase as the number of turns and induced voltage are directly proportional to each other.
Therefore, if the number of turns in the coil of this dynamo is doubled, then the voltage in the bulb will also be doubled.

Multiple choice physics magnetism and electromagnetism electric generator ac generator magnetic effects of electric current

What is a commutator?

  1. A device that reverses the direction of flow of current through a circuit is called a commutator.

  2. A device that flows in the direction of flow of current through a circuit is called a commutator.

  3. A device that reverses the direction of flow of voltage through a circuit is called a commutator.

  4. All
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A device that reverses the direction of flow of current through a circuit is called a commutator. In electric motors, the split ring acts as a commutator.

Multiple choice physics effects of electric current electric generator ac generator magnetic effects of electric current

Fleming's left and right hand rule are used in

  1. DC motor and AC motor

  2. DC generator and AC motor

  3. DC motor and DC generator

  4. both rules are same, any one can be used
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

DC motor is a device which coverts electrical energy into mechanical energy. It employs Fleming's left hand rule.
DC generator converts mechanical energy into electrical energy in from of DC. It employs Fleming's right hand rule.

Multiple choice physics magnetism and electromagnetism electric generator ac generator magnetic effects of electric current

If a loop in a basic dc generator suddenly begins rotating at a faster speed, the induced voltage

  1. remains unchanged

  2. reverses polarity

  3. increases

  4. decreases
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Induced voltage in the rotating loop is given by   $\mathcal{E} = NBAw \ \sin wt$
where $N$ is the number of turns in the loop, $A$ is the area of the loop, $B$ is the magnetic field and $w$ is the angular speed of rotation of loop.
$\implies \ \mathcal{E}\propto w$
So, the induced voltage increases on increasing the speed of rotation of loop.

Multiple choice physics effects of electric current electric generator ac generator magnetic effects of electric current

Which of the following statement is correct?

  1. AC generator generates a higher voltage

  2. DC generator generates a higher voltage

  3. AC generator has a permanent magnet whereas a DC generator has an electromagnet

  4. There is a split-ring commutator in a DC generator but not in an AC generator
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The voltage generated by a generator , whether AC or DC , depends upon the rate of change of flux linked with coil .

Both type of generators usually have permanent magnets .
There is a split- ring commutator in a DC generator but an AC generator has slip - rings .
Option D is correct .

Multiple choice physics effects of electric current electric generator ac generator magnetic effects of electric current

The current produced in a generator armature is AC because

  1. the magnetic field reverses at intervals

  2. The current in the field coils is AC

  3. the rotation of the armature causes the field through it to reverse

  4. the commutator feeds current into it in opposite directions every half cycle
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

After every half rotation, the direction of e.m.f reverses and hence current changes its direction after a regular interval.

Hence, Answer-(C)